RE: [wss] SwA Interoperability Document

> *4. Signing a child element within an attachment that happens to be
XML.
> This scenario will involve the use of an XPath transform at a minimum.

Although the WSS SwA doesn't indicate currently, it should also be
possible to reference an element using an ID ref like cid:<id>#<ref>

That being said, the XML attachment may not contain any wsu:Id to
reference. 

--ms